1. Field of the Invention
The present invention relates generally to oil pressure control systems for automatic transmissions employed in vehicles, and more particularly, to a system for controlling oil pressure applied through an oil hydraulic control circuit to engaging friction elements or other operating elements in an automatic transmission provided in a vehicle.
2. Description of the Prior Art
There has been commonly used in vehicles an automatic transmission which is coupled with an engine provided in the vehicle and includes a torque converter comprising a pump impeller, a turbine runner and a stator, and a power transmitting gear arrangement including a speed change mechanism having engaging friction elements, such as clutch and brake elements, operating with oil pressure. The automatic transmission employed to be coupled with the engine in the vehicle is usually accompanied with an oil pressure control system including an oil hydraulic control circuit. The oil hydraulic control circuit comprises a line pressure producing portion including a regulator valve which is operative to produce line pressure by adjusting oil pressure obtained from an oil pump and an operation oil pressure producing portion including a manual shift valve caused manually to perform shifting range change, a plurality of shift valves each producing operating oil pressure based on the line pressure and supplying selectively the engaging friction elements in the power transmitting gear arrangement with the operating oil pressure and a plurality of solenoid valves operative to change operating conditions of the shift valves, respectively.
On the occasion of speed change control performed in the automatic transmission, in a control unit provided in the oil pressure control system, engine road and vehicle speed actually detected are collated with shifting lines in a predetermined speed change characteristic map stored in a memory device in the control unit to detect a shifting condition which is satisfied on that occasion, and a driving signals are supplied from the control unit to the solenoid valves provided in the operating oil pressure producing portion in response to the shifting condition detected by the control unit so as to change operating conditions of the shifting valves. With the change in the operating conditions of the shifting valves, one or more of the engaging friction elements are caused to be in engagement or disengagement and thereby a speed change operation is carried out in the automatic transmission.
In respect of the oil pressure control system, there have been proposed to provide the line pressure producing portion with a solenoid valve and to control the length of each period of time in which the solenoid valve is kept in its ON state is controlled to vary minutely the line pressure obtained from the line pressure producing portion as disclosed in, for example, the Japanese patent application published before examination under publication number 54-2349.
Further, it has been also proposed an oil pressure control system in which the line pressure obtained from the line pressure producing portion is varied in response to each speed change condition so that abrasion on the engaging friction elements is reduced and shocks caused by the speed change are restrained.
In the oil pressure control system for the automatic transmission as described above, a pressure control variable used for controlling the line pressure is usually determined in response to, for example, actual opening degree of a throttle valve so as not to reflect dispersion in characteristic and aged deterioration of each automatic transmission. Therefore, it is feared that the pressure control variable used for controlling the line pressure comes to be unfit for the speed change operation carried out in the automatic transmission due to the dispersion in characteristic and the aged deterioration of each automatic transmission so that the duration of speed change is caused to be inappropriate.
For avoiding the above mentioned problems, there has been proposed another oil pressure control system for the automatic transmission such as disclosed in, for example, the Japanese patent application published before examination under publication number 63-3183. In such an oil pressure control system, target durations of speed change corresponding respectively to various speed change operations expected to be carried out in the automatic transmission are previously stored in a memory device contained in a control unit and the duration of speed change detected actually in the automatic transmission is compared with the target duration of speed change corresponding to the duration actually detected so as to produce a comparison output with which the pressure control variable used for controlling the line pressure is revised. However, in the case of the oil pressure control system for the automatic transmission wherein the pressure control variable used for controlling the line pressure is revised based on the output of comparison between the duration of speed change and the target duration of speed change stored in the memory device contained in the control unit, since the pressure control variable used for controlling the line pressure is revised regardless of the actual opening degree of the throttle valve, it is feared that the pressure control variable is not respondent to the operating condition of the engine with which the automatic transmission is coupled or the traveling condition of the vehicle.
For dealing with the above described problems, it is considered to store a large number of target durations of speed change corresponding respectively to various opening degrees of the throttle valve in the memory device in the control unit so that each target duration of speed change is read from the memory device in response to the actual opening degree of the throttle valve to be utilized to revise the pressure control variable used for controlling the line pressure. However, in such a case, since lots of labor are necessary to store the large number of target durations of speed change in the memory device and the memory device is required to have large capacity for memorization, the oil pressure control system for the automatic transmission comes to be expensive.